The present invention relates to a cover body for an air bag apparatus which is integrally formed with an internal panel of a vehicle. This invention is applicable particularly, but not exclusively, to a cover body for an air bag apparatus to be used for a front passenger seat, the cover body being formed integrally with an instrument panel.
Instrument panels located below the front window of a vehicle have been designed as instrument panels with hidden seamlines, which have an integrated air bag apparatus comprising an air bag and an inflator stored inside the instrument panel. Within the instrument panel, a lower portion of the air bag apparatus is fixed to a steering member disposed along the direction of the width of a vehicle, and an upper portion thereof is attached to a pair of attaching members consisting of a back and front member which are formed integrally with a rear of an instrument panel.
Japanese Patent Provisional Publication 2001-39254 discloses an air bag apparatus wherein a door portion is formed in a portion of an instrument panel directly covering an air bag between a pair of attaching members by the instrument panel itself being sectioned by a tear seam formed in a closed-loop shape on a back surface thereof.
By forming the door portion in the instrument panel itself in this manner, an outer appearance of the instrument panel is maintained in an attractive state, as compared to forming an opening in the instrument panel and emplacing a separately formed lid which closes the opening and opens upon deployment of an air bag. An aluminum backing member is formed on a back inner side of the door portion, the backing member extending past the tear seam to beyond the door portion. When the air bag inflates, the inflation force of the air bag pushes the entire door portion by pushing through the backing member, and the door portion is torn along the tear seam and the door portion is torn away. The torn away door portion swings outside to a front outer side of the instrument panel with a portion of the backing member directly under the tear seam acting as a hinge portion, and from the opening formed by the severed door portion, the air bag inflates into the interior of the vehicle.
However, according to the above related art, since the backing member joined to the back side of the door portion of the instrument panel is metal (in the above example, aluminum), the inflation force of the air bag does not directly act on a portion of the tear seam which is covered by the backing member. That is, as the backing member is formed from metal and is therefore not soft, it is not possible for the inflation force of the air bag to directly act on the portion of the tear seam covered by the backing member. Rather, the tear seam is torn by force being exerted through the entire metal backing member, which produces an indirect pushing effect against the entire door portion. Therefore, it is necessary for the air bag apparatus to use an inflator for inflating the air bag which is large-size and high-powered, in order for the tear seam to tear properly with certainty. Such a solution is a disadvantage with respect to weight and cost.
To solve this problem, Japanese Patent Provisional Publication 2002-347556 by the present applicant discloses an air bag apparatus wherein the backing member is made from a plastic material. Specifically, the backing member is made from a soft resin material, and the location of the backing member directly under the tear seam receives the inflation force of an air bag, is pushed up, and easily deforms, thus allowing a pushing-up force to act directly against the tear seam. In more detail, a thermoplastic olefin elastomer resin is used as material for the backing member, the resin having a flexual elastic modulus of 6000 kgf/cm2 or less, preferably 4500 kgf/cm2 or less, and as an embodiment, 2800 kgf/cm2.
Japanese Patent Provisional Publication (Heisei) 9-109816 discloses an arrangement where an instrument panel doubles as a passenger-side air bag cover, and a reinforcing portion is disposed on a back side thereof. Materials for the instrument panel and reinforcing portion in this combination are preferably a polyolefin and olefin elastomer, respectively. Specifically, the instrument panel is polypropylene resin which has an elongation of more than 300%, a flexual elastic modulus of more than 20000 kgf/cm2, a temperature of deflection under load of more than 120° C., and an Izod impact strength at 23° C. of more than 15 kg·cm/cm. The reinforcing portion is made of an olefin elastomer which has a flexual elastic modulus of more than 3000 kgf/cm2, and an Izod impact strength at −40° C. of more than 20 kg·cm/cm.
Japanese Patent Provisional Publication (Heisei) 8-48204 discloses a two-layer cover for a gas bag module where hardness of the first layer is in a range of approximately Shore D hardness 32 to 43, and hardness of the second layer is in an approximate range of Shore A hardness 45 to 80.
Also, in an instrument panel with hidden seamlines having an air bag apparatus integrally formed therewith as in the above examples, a bent groove portion is formed in the backing member which is directly under a location to constitute the hinge of the door portion.
A backing member made of thermoplastic polyurethane foam is disposed on a back side of a door portion, and extends beyond a tear seam and beyond the door portion. Due to the inflation force during deployment of an air bag, the entire door portion is pushed up by force exerted through the backing member, and the door portion is torn along the tear seam and is completely torn away. The torn away door portion swings outward to an outside of the instrument panel with a portion of the backing member directly under the tear seam acting as a hinge, and from an opening formed by the severed door portion, the air bag expands into the interior of a vehicle.